In recent years, a semiconductor device which uses an amorphous oxide semiconductor thin film has attention. The thin film of this type can be deposited at low temperature, and has optical transparency in regard to visible light because an optical band gap thereof is large. For these reasons, a flexible transparent TFT (thin film transistor) or the like can be formed on a plastic substrate, a film substrate or the like.
For example, Japanese Patent Application Laid-Open No. 2002-076356 describes a technique concerning a TFT in which an oxide film including Zn—O as main components is used as a channel layer.
Further, “Nature, Vol. 432, 25 Nov. 2004 (488-492)” describes a technique concerning a TFT in which an amorphous oxide film including In, Zn and Ga and formed at room temperature is used as a channel layer.
Furthermore, “Journal of Non-Crystalline Solids, 352 (2006) 2311” describes that an oxide thin film including indium oxide as a main component and formed at room temperature is used as a channel layer (active layer) of a TFT.
Moreover, U.S. Patent Publication No. 2006/0108636 describes a TFT in which an amorphous oxide film including In, Zn and Ga and formed at room temperature is used as a channel layer. In addition, U.S. Patent Publication No. 2006/0108636 describes a technique concerning a TFT in which an impurity such as Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, P or the like has been added to the channel layer.
In the TFT described in “Nature, Vol. 432, 25 Nov. 2004 (488-492)”, although an S value is relatively large, i.e., about 2V/decade, field effect mobility is high, i.e., 6 cm2/Vs to 9 cm2/Vs. For this reason, it is expected that this TFT is applied to a channel matrix which is desired for a flat display apparatus using a liquid crystal, an electroluminescence or the like. However, according to the knowledge of the present inventors, in this TFT, the characteristic thereof significantly changes according to an atomic composition ratio (also called an atomic ratio, or an atomic composition percentage) of main component metal elements of an amorphous oxide film used as the channel layer.
On the other hand, U.S. Patent Publication No. 2006/0108636 describes that a carrier concentration is controlled by adding the impurity element to the channel layer, and the TFT having a large current on/off ratio is thus acquired. However, U.S. Patent Publication 2006/0108636 is silent about an effect of addition of the impurity in a case where an atomic composition ratio of main component metal elements is changed.
Besides, in the TFT described in “Journal of Non-Crystalline Solids 352 (2006) 2311”, field effect mobility is 10 cm2/Vs to 140 cm2/Vs and an S value is 0.09V/decade to 5.6V/decade depending on a material of a gate insulation film.
However, according to the knowledge of the present inventors, in the relevant TFT, since environmental stability of an In—O film formed at room temperature is low, the resistivity of the In—O film significantly changes if it is left in atmosphere. For example, if the In—O film is left in atmosphere at temperature 20° C. and humidity 50% for one month, deterioration of the resistivity from one digit to two digits is observed. Also, such deterioration of resistivity is similarly observed in an oxide semiconductor using Zn—O as a main component described in Japanese Patent Application Laid-Open No. 2002-076356.
The present invention aims to solve such problems as described above, and an object thereof is to provide an amorphous oxide excellent in transistor characteristics such as field effect mobility, an S value and the like, excellent in environmental stability, and having a large atomic composition ratio margin (design flexibility), and aims to provide a filed effect TFT having the relevant amorphous oxide.